Question

What is the difference between a point on the Earth's grid system and one on the celestial system?

Answer 1

On the Earth's grid system, points are found using two imaginary lines. These imaginary lines are 1) parallels of latitude and 2) meridians of longitude. 

On the Celestial system, declination and right ascension or hour angles is used in lieu of latitude and longitude, respectively.

The latitude or declination both measure north and south portion of the Earth's grid system or the Celestial system.

The longitude or right ascension both measure the east and west portion of the Earth's grid system or the Celestial system.